1. Field of the Invention
The present invention relates to an LC noise filter.
2. Description of the Prior Art
With recent development in electronic techniques, various types of electronic circuits are currently utilized in many fields of art. It is thus desirable that such electronic circuits can operate stably and reliably without being affected by any external disturbance.
Actually, however, such electronic circuits would be frequently disturbed directly or indirectly by external noise. This may frequently cause malfunction therein.
In particular, there are many electronic circuits which utilize a switching regulator as DC source. It is thus frequent that large noises having various frequency components are created on the source line of a switching regulator due to transient current on switching or due to variations of load from the switching operation of a digital integrated circuit (IC) used therein. These noises may adversely affect the other circuits in the same instrument through the source line or through radiation to create malfunction or reduction of S/N ratio. In addition, the noises may create malfunction in the other electronic instruments used adjacent to the noise-source instrument.
In order to remove the noises, generally, the electronic circuits include various types of noise filters. Particularly, there are many types of electronic high-performance instruments more recently used in the art. It is thus desired to develop a small-sized and high-performance noise filter which can reliably eliminate any possible noise.
One of noise filters developed for such a purpose is an LC noise filter shown in FIG. 22. As seen from FIG. 22A, this noise filter comprises a core 10 and a pair of windings 12 and 14 wound around the core 10. A capacitor 16 or 18 is connected in parallel between the windings 12 and 14 at the respective ends thereof.
In such a prior art noise filter, however, separately formed windings 12, 14 and capacitors 16, 18 are required. This leads to increase of the noise filter in size and manufacturing cost, in comparison with the recent desire of reduction in size and weight.
In addition, as seen from FIG. 22B, the prior art noise filter is of lumped constant type. Such a type of noise filter cannot reliably remove various types of noises entering the circuit, and particularly common-mode noises created in switching surge or normal-mode noises corresponding to ripple. If for the purpose of elimination of such noises, the capacity or inductance in the capacitor or coil is increased, there will then arise such a problem that a ringing is created.